Action (firearms) in the context of Firearms

A shotgun (also known as a scattergun, peppergun, or historically as a fowling piece) is a long-barreled firearm designed to shoot a straight-walled cartridge known as a shotshell, which discharges numerous small spherical projectiles called shot, or a single solid projectile called a slug. Shotguns are most commonly used as smoothbore firearms, meaning that their gun barrels have no rifling on the inner wall, but rifled barrels for shooting sabot slugs (slug barrels) are also available.

Shotguns come in a wide variety of calibers and gauges ranging from 5.5 mm (.22 inch) to up to 5 cm (2.0 in), though the 12-gauge (18.53 mm or 0.729 in) and 20-gauge (15.63 mm or 0.615 in) bores are by far the most common. Almost all are breechloading, and can be single barreled, double barreled, or in the form of a combination gun. Like rifles, shotguns also come in a range of different action types, both single-shot and repeating. For non-repeating designs, over-and-under and side-by-side break action shotguns are by far the most common variants. Although revolving shotguns do exist, most modern repeating shotguns are either pump action or semi-automatic, and also fully automatic, lever-action, or bolt-action to a lesser extent.

A sniper rifle is a high-precision, long-range rifle. Requirements include high accuracy, reliability, mobility, concealment, and optics, for anti-personnel, anti-materiel and surveillance uses by military snipers. The modern sniper rifle is a portable shoulder-fired rifle with either a bolt action or semi-automatic action, fitted with a telescopic sight for extreme accuracy and chambered for a high-ballistic performance centerfire cartridge.

The Heckler & Koch MP5 (German: Maschinenpistole 5, lit. 'Submachine gun 5') is a submachine gun developed in the 1960s by German firearms manufacturer Heckler & Koch. It uses a similar modular design to the Heckler & Koch G3, and has over 100 variants and clones, including selective fire, semi-automatic, suppressed, compact, and even marksman variants. The MP5 is one of the most widely used submachine guns in the world, having been adopted by over forty nations and numerous militaries, police forces, intelligence agencies, security organizations, paramilitaries, and non-state actors.

Attempts at replacing the MP5 by Heckler & Koch began in the 1980s, but despite functional prototype weapons having promising performance, a formal successor did not enter commercial production until 1999, when Heckler & Koch developed the UMP. However, despite being more expensive, the MP5 remained the more successful of the two designs, because of its preexisting widespread use, design familiarity, and lower recoil due to its roller-delayed action as opposed to the UMP's straight blowback action.

A repeating rifle is a single-barreled rifle capable of repeated discharges between each ammunition reload. This is typically achieved by having multiple cartridges stored in a magazine (within or attached to the rifle) and then fed individually into the chamber by a reciprocating bolt, via either a manual or automatic action mechanism, while the act of chambering the round typically also recocks the hammer/striker for the following shot. In common usage, the term "repeating rifle" most often refers specifically to manual repeating rifles (e.g. lever-action, pump-action, bolt-action, etc.), as opposed to self-loading rifles, which use the recoil, gas, or blowback of the previous shot to cycle the action and load the next round, even though all self-loading firearms are technically a subcategory of repeating firearms.

Repeating rifles were a significant advance over the preceding single-shot, breechloading rifles when used for military combat, as they allowed a much greater rate of fire. The repeating Henry rifle was used by the infantry and Spencer rifle was used by the cavalry during the American Civil War and the subsequent American Indian Wars, and the first repeating air rifle to see military service was the Windbüchse rifle.

In firearms, the cylinder is the cylindrical, rotating part of a revolver containing multiple chambers, each of which is capable of holding a single cartridge. The cylinder rotates (revolves) around a central axis in the revolver's action to sequentially align each individual chamber with the barrel bore for repeated firing. Each time the gun is cocked, the cylinder indexes by one chamber (for five-chambers, by 72°, for six-chambers, by 60°, for seven-chambers, by 51.43°, for eight-chambers, by 45°, for nine-chambers, by 40°, and for ten-chambers, by 36°). Serving the same function as a rotary magazine, the cylinder stores ammunitions within the revolver and allows it to fire multiple times, before needing to be reloaded.

Typically revolver cylinders are designed to generally hold six cartridges (hence revolvers sometimes are referred to as six-guns or six-shooters), but some small-frame concealable revolvers such as the Smith & Wesson Model 638 have a 5-shot cylinder, due to the smaller overall size and limited available space. The Nagant M1895 revolver has a 7-shot cylinder, the Webley-Fosbery Automatic Revolver has an 8-shot cylinder in .38 ACP, the LeMat Revolver has a 9-shot cylinder, and the Smith & Wesson Model 617 has a 10-shot cylinder in .22 Long Rifle.

A derringer or deringer is a small handgun that is neither a revolver, semi-automatic pistol, nor machine pistol. It is not to be confused with mini-revolvers or pocket pistols, although some later derringers were manufactured with the pepperbox configuration. The modern derringer is often multi barreled, and is generally the smallest usable handgun of any given caliber and barrel length due to the lack of a moving action, which takes up more space behind the barrel. It is frequently associated with discreet use by women because it is easily concealable in a purse or a stocking.

The original Philadelphia Deringer was a muzzleloading caplock single-shot pistol introduced in 1825 by Henry Deringer. In total, approximately 15,000 Deringer pistols were manufactured. All were single-barrel pistols with back-action percussion locks, typically .41 caliber with rifled bores and walnut stocks. Barrel length varied from 1.5 to 6 in (38 to 152 mm), and the hardware was commonly a copper-nickel alloy known as "German silver".

In firearm designs, the term single-shot refers to guns that can hold only a single round of ammunition inside and thus must be reloaded manually after every shot. Compared to multi-shot repeating firearms ("repeaters"), single-shot designs have no moving parts other than the trigger, hammer/firing pin or frizzen, and therefore do not need a sizable receiver behind the barrel to accommodate a moving action, making them far less complex and more robust than revolvers or magazine/belt-fed firearms, but also with much slower rates of fire.

The history of firearms began with muzzleloading single-shot firearms such as the hand cannon and arquebus, then multi-barreled designs such as the derringer appeared, and eventually many centuries passed before breechloading repeating firearms became commonplace. Although largely disappeared from military usage due to insufficient firepower, single-shot firearms are still produced by many manufacturers in both muzzleloading and cartridge-firing varieties, from zip guns and ultra-concealable pocket pistols to the highest-quality hunting and match rifles.

A lever action is a type of action for repeating firearms that uses a manually operated cocking handle located around the trigger guard area (often incorporating it) that pivots forward to move the bolt via internal linkages, which will feed and extract cartridges into and out of the chamber, and cock the firing pin mechanism. This contrasts to other type of repeating actions such as the bolt-action, pump-action, semi-automatic, fully automatic, and/or burst mode actions. A firearm using this operating mechanism is colloquially referred to as a levergun.

Most lever-action firearms are rifles, but some lever-action shotguns and a few pistols have been made. The Winchester Model 1873 rifle is one of the most famous lever-action firearms, but many manufacturers (notably Henry Repeating Arms and Marlin Firearms) also produce lever-action rifles. Colt produced the 6403 lever-action Colt-Burgess rifles from 1883 until 1885 and Mossberg formerly produced the Model 464 rifle.

A gunstock or often simply stock, the back portion of which is also known as a shoulder stock, a buttstock, or simply a butt, is a part of a long gun that provides structural support, to which the barrel, action, and firing mechanism are attached. The stock also provides a means for the shooter to firmly brace the gun and easily aim with stability by being held against the user's shoulder when shooting the gun, and helps to counter muzzle rise by transmitting recoil straight into the shooter's body.

The tiller of a crossbow is functionally the equivalent of the stock on a gun.

A repeating firearm or repeater is any firearm (either a handgun or long gun) that is designed for multiple, repeated firings before the gun has to be reloaded with new ammunition.

Unlike single-shot firearms, which can only hold and fire a single round of ammunition, a repeating firearm can store multiple cartridges inside a magazine (as in pistols, rifles, or shotguns), a cylinder (as in revolvers), or a belt (as in machine guns), and uses a moving action to manipulate each cartridge into and out of the battery position (within the chamber and in alignment with the bore). This allows the weapon to be discharged repeatedly in relatively quick succession, before manually reloading the ammunition is needed.

Break action is a type of firearm action in which the barrel(s) are hinged much like a door and rotate perpendicularly to the bore axis to expose the breech and allow loading and unloading of cartridges. A separate operation may be required for the cocking of a hammer to fire the new round. There are many types of break-action firearms; break actions are universal in double-barreled shotguns, double-barreled rifles, combination guns, and are commonly found in single shot pistols (especially derringers), rifles, shotguns, including flare guns, grenade launchers, air guns, and some older revolver designs. They are also known as hinge-action, break-open, break-barrel, break-top, or, on old revolvers, top-break actions.

A semi-automatic shotgun is a repeating shotgun with a semi-automatic action, i.e. capable of automatically chambering a new shell after each firing, but requires individual trigger-pull to manually actuate each shot. Semi-automatic shotguns use gas operation, blowback, or recoil operation to cycle the action, eject the empty shell, and load another round. The first semi-automatic shotgun was the Automatic-5 by Browning.

Many semi-automatic shotguns also provide an optional manual means of operation such as by pump action or a charging handle.

Bolt action is a type of manual firearm action that is operated by directly manipulating the turn-bolt via a bolt handle, most commonly placed on the right-hand side of the firearm (as most users are right-handed). The majority of bolt-action firearms are rifles, but there are also some variants of shotguns and handguns that are bolt-action.

Bolt action firearms are generally repeating firearms, but many single-shot designs are available as well, particularly in shooting sports where single-shot firearms are mandated, such as most Olympic and ISSF rifle disciplines.

An automatic firearm or fully automatic firearm (to avoid confusion with semi-automatic firearms) is a self-loading firearm that continuously chambers and fires rounds when the trigger mechanism is actuated. The action of an automatic firearm is capable of harvesting the excess energy released from a previous discharge to feed a new ammunition round into the chamber, and then igniting the propellant and discharging the projectile (either bullet, shot, or slug) by delivering a hammer or striker impact on the primer.

If both the feeding and ignition procedures are automatically cycled, the weapon will be considered "fully automatic" and will fire continuously as long as the trigger is kept depressed and the ammunition feeding (either from a magazine or a belt) remains available. In contrast, a firearm is considered "semi-automatic" if it only automatically cycles to chamber new rounds (i.e. self-loading), but does not automatically fire off the shot unless the user manually resets (usually by releasing) and re-actuates the trigger, so only one round gets discharged with each individual trigger-pull. A burst-fire firearm is an "in-between" of fully and semi-automatic firearms, firing a brief continuous "burst" of multiple rounds with each trigger-pull, but then will require a manual re-actuation of the trigger to fire another burst.

A magazine, often simply called a mag, is an ammunition storage and feeding device for a repeating firearm, either integral within the gun (internal or fixed magazine) or externally attached (detachable magazine). The magazine functions by holding several cartridges within itself and sequentially pushing each one into a position where it may be readily loaded into the barrel chamber by the firearm's moving action. The detachable magazine is sometimes colloquially referred to as a "clip," although this is technically inaccurate since a clip is actually an accessory device used to help load ammunition into the magazine or cylinder of a firearm.

Magazines come in many different shapes and sizes from integral tubular magazines on lever-action and pump-action rifles and shotguns that may hold more than 5 rounds to detachable box magazines and drum magazines for automatic rifles and light machine guns that may hold more than 50 rounds. Various jurisdictions ban what they define as "high-capacity magazines".